Dynamic microphone



June 28, 1966 H. F. MoslER, JR

DYNAMIC MICROPHONE 2 Sheets-Sheet. 1

Filed Oct, l, 1962 i www, ws da t 0 m fir @Me/.w Ehm? n Im @.5 0 t wwwya z v0 9, ||||1. ww

June 28. 1966 H. F. MoslER, JR

DYNAMIC MICROPHONE 2 Sheets-Sheet 2 Filed 0oz, 1, 1962 ,Burmeister{infr/mer United States Patent O 3,258,543 DYNAMIC MICROPHONE Harold F.Mosier, Jr., Buchanan, Mich., assigner to Electro-Voice, Incorporated,Buchanan, Mich., a corporation of Indiana Filed Oct. 1, 1962, Ser. No.227,498 22 Claims. (Cl. 179-1155) The present invention relates toelectromagnetic transducers, and particularly to microphones andearphones of the moving coil type. The invention also relates to methodsfor making electroacoustical devices.

Moving coil type microphones and earphones are well known, and thetextbook, Acoustical Engineering, by Harry F. Olson, D. Van Nostrand &Co., Princeton, New Jersey, 1957, describes and analyzes the operationof such microphones at pages 260 through 263. Such microphones andearphones generally employ a case which has an opening closed by avibratal diaphragm. The vibratal diaphragm of the microphone or earphonecarries a coil which is disposed within a gap in a magnetic circuit. Insuch a microphone, movement of the diaphragm, and hence the coil,referred to as a voice coil, generates an electric current in the voicecoil, since it is cutting magnetic lines of force. In such an earphone,an alternating electric current ilowing through the coil vibrates thediaphragm.

Since the magnitude of the current generated in the voice coil of amicrophone responsive to vibration of the diaphragm is proportional tothe ilux density in the magnetic gap, the magnetic gap is made as smallas practical, and the voice coil is carefully centered in the magneticgap. The same considerations also apply to construction of earphones. Iti-s one of the objects of the present invention to provide a structurefor use in microphones and earphones which facilitates centering of thecoil within the magnetic gap.

In a microphone, the voltage generated in the voice coil is .alsoproportional to the velocity of the voice coil Within the magnetic gap.As a result, the microphone will have a uniform sensitivity throughoutits frequency range if the velocity of the voice coil is madeindependent of frequency. The velocity of the voice coil may be madeindependent of frequency if it is essentially resistance controlledrather than mass controlled.

Resistance control of the voice coil is conventionally obtained bydamping the diaphragm which carries the voice coil. For this purpose,the flow of air behind the diaphragm is restricted by silk or feltresistance members, such as shown in Patent No. 3,014,099 to Fiala,entitled, Electroacoustic Transducer. It has often proven difficult toprovide the proper material to produce the desired resistance for aparticular microphone. Further, felt and silk resistance members aresubject to deterioration.

Slots or air gaps have also been employed prior to the present inventionto provide the desired resistance control of the diaphragm, as shown bythe Fiala patent and Patent No. 2,773,130 to Olson, entitled, AcousticalResistance for Pressure Type Microphones. It has been dicult in practiceto control these slots or air gaps to provide the required damping.

It is therefore a further object of the present invention to provide animproved dynamic microphone structure which utilizes slots or air gapsto control a diaphragm, and to provide such a microphone structure whichmay be readily reproduced.

It is also an object of the present invention to provide a dynamicmicrophone structure which may be assembled more readily than priormicrophone structures, is less costly to assemble, and possesses moreuniform operating characteristics when produced under mass productiontechniques.

It is also an object of the present invention to provide 3,258,543Patented June 28, 1966 an earphone structure which may be assembled morereadily than prior earphone structures, is less costly to assemble, andwhich produces more uniform operating characteristics when produced bymass production techniques.

It is a further object of the present invention to provide a method ofmaking electroacoustical devices which simplies assembly techniques,reduces costs, and increases reliability, particularly reducing thenumber of units rejected due to coils scraping Within the magnetic gapsof the units.

Further objects of the present invention will become apparent from afurther consideration of this specication, particularly when viewed inthe light of the drawings, in which:

FIGURE 1 is an exploded view of a microphone structure constructedaccording to the teachings of the present invention and illustrated inFIGURES 1 through 5;

FIGURE 2 is a sectional view of the microphone structure taken along theline 2-2 of FIGURE 3;

FIGURE 3 is a sectional view of the microphone structure taken along theline 3-3 of FIGURE 2;

FIGURE 4 is a sectional view of the microphone taken along the line 4-4of FIGURE 3;

FIGURE 5 is a fragmentary sectional view of the microphone taken alongthe line 5-5 of FIGURE 4, the external casing being omitted;

FIGURE 6 is a rear elevational view (partly broken away and in section)of a modified microphone construction according to the presentinvention;

FIGURE 7 is a schematic electrical circuit diagram illustrating themechanical network which constitutes the microphone structure inelectrical symbols;

FIGURE 8 is a fragmentary sectional View of another embodiment of thepresent invention taken on a plane similar to that of FIGURE 5;

FIGURE 9 is a vertical sectional view of an earphone constructedaccording to the teachings of the present invention;

FIGURE 10 is a front elevational view of .the earphone of FIGURE 9;

FIGURE 11 is a rear elevational view of the earphone of FIGURES 9 andl0; and

FIGURE 12 is a vertical sectional 'View of a mold suitable formanufacturing the earphone of FIGURES 9 through 11.

In the embodiment illustrated in FIGURES 1 through ICC ` 5, themicrophone structure has a diaphragm 10, which is mounted on asupporting ring 12 and carries a voice coil 14. The supporting ring 12also mounts a cylindrical magnet 16 and a U-shaped magnetic pole piece18. In addition, a pair of lugs 20 are supported on the surface of thesupporting ring 12 opposite the diaphragm 10.

Referring to FIGURES 2 through 5, it will be seen that the supportingring 12 has a ferromagnetic part 22 and a nonmagnetic part 24. Theferromagnetic part 22 is in the form of a circular disc, or plate,except for an indentation 26 at its periphery. The ferromagnetic part ordisc 22 has a central cylindrical opening 28 for accommodating themagnet 16, and a plurality of apertures 30 disposed at the same distancefrom the center of the ferromagnetic disc 22 and spaced at equalintervals about the periphery of the disc, lalthough it is not requiredthat the apertures be equally spaced or at a common distance from thecenter.

The nonmagnetic part 24 has a disc portion 32 disposed on one side ofthe ferromagnetic part or disc 22 and a ring portion 34 disposed on theother side of the disc 22. The disc portion 32 and the ring portion 34are interconnected by rod portions 36 which extend through the apertures30, and the disc portion 32, ring portion 34 and rod portions 36 arepreferably constructed of an integral mass of the same material, such asplastic. The

disc portion 32 of the nonmagnetic part 24 is also provided With acentral cylindrical opening 38 which is coaxial with the opening 28 inthe ferromagnetic part or disc of the supporting ring 12. The diameterof the opening 38 is equal to the outer diameter of the magnet 16, butthe disc portion 32 is provided with a coaxial recess 39 and a pluralityof slots 44 which extend from the recess 39 parallel to the axis of themagnet 16, thus forming ribs 40. The magnet 16 is in abutment with theribs 40 of the opening 38 and the cylindrical surface 42 of the magnet16 and slots 44, thereby form arcuate spaced passages from the surfaceof the recess 39 through the disc portion 32, as illustrated in FIGURE4. In the particular construction here described, the passages formed bythe slots 44 are equally spaced about the opening 38 forming four slots44, although it is to be understood that the slots 44 need not beequally spaced and may be more or fewer in number.

The ribs 40 have the double function of securing and centering themagnet 16 on the supporting ning 12 and providing the proper clearancefor the slots 44. The slots provide a resistance and an inductivecomponent for damping the diaphragm, as will be hereinafter explained.

The magnet 16 extends from the supporting ring 12 to the pole piece 18.The pole piece 18 has a pair of legs 46 and 48 which extend from a crossmember 50, the cross member 50 abutting the end of the magnet 16opposite the supporting ring 12. The legs 46 and 43 extend into abutmentwith the ferromagnetic disc 22 which has a diameter greater than thediameter of the disc portion 32 of the nonmagnetic part 24 of thesupporting ring 12. The legs 46 and 48 abut the edges of the discportion 32 and abut the ferromagnetic disc 22 at their ends. The magnet16 extends into the opening 28 in the ferromagnetic disc 22, thusforming a circular gap 52 between the magnet surface 42 and theconfronting opening 28 of the ferromagnetic disc 22. The voice coil 14is translat ably disposed Within this gap 52 along the axis of themagnet 16. The magnetic circuit includes the magnet 16, the cross member50, legs 46 and 48, ferromagnetic disc 22, and the gap 52.

The diaphragm has .a circular perimeter with a diameter equal to theinner diameter of the ring portion 34 of the supporting ring 12. Thediaphragm 10 has a flat ring-shaped penipheral portion 54 which iscemented on the confronting surface of the ferromagnetic disc 22, and iscentered coaxially about the central axis of the openings 28 and 38 bythe inner perimeter of the ring portion 34 of the supporting ring 12.The diaphragm 10 has a plurality of utes 56 disposed in a circular bandcoaxially within a circular ilange portion 57 which extends from theperipheral flat portion 54. The band of flutes 56 and flange portion 57space a circular central portion 58 of the diaphragm from theferromagnetic disc 22. The circular central portion 53 of the diaphragm10 is in the form of a dome protruding outwardly from the ferromagneticdisc 22, and the portion of the diaphragm within the circular flangeportion 57, including the domed central portion 58 and utes 56, is freeto vibrate relative to the disc 22. The coil 14 also is circular in formand mounted on the surface of the diaphragm 10 confronting theferromagnetic disc 22. The coil 14 has a diameter selected to permit itto be freely translated within the gap 52. The coil 14 is cemented ontothe diaphragm 10 about the perimeter of the domed central portion 58thereof, and the ends of the coil 14 are brought out through apertures60 in the diaphragm 10 at the interface between the domed centralportion 58 and `the portion having the flutes 56.

The nonmagnetic part 24 of the supporting ring 12 has a flange 62 whichfills the indentation 26 of the ferromagnetic part 22, thereby providinga completely circular periphery to the supporting ring 12. A groove 64normal to the surface of the ferromagnetic disc 22 is disposed centrallyof the flange 62 to permit the ends of the coil 14 to traverse from theface of the diaphragm 10 to the lugs 20 mounted on the nonmagnetic part24 of the supporting ring 12 on the side of the ferromagnetic disc 22opposite the diaphragm 10. The ring portion 34 of the supporting ring 12is also provided with a gap 68 for the ends of the coil 14.

In the manufacture of a microphone structure as described in FIGURES lthrough 5, the nonmagnetic part 24 of the supporting ring 12 is moldedas a unit on the ferromagnetic part 22 thereof, the ring portion 34, rodportions 36, and disc portion 32 being molded in a single operation.This same molding operation forms the ribs 40 on the opening 38. Themicrophone structure is assembled by inserting the cylindrical magnetinto the opening 38 and the opening 28, and thereafter mount-ing thepole piece 18 in position. A layer of cement 70 may be disposed betweenthe pole piece 18 and the end of the magnet 16, and cement layers 72 and'74 may be disposed between the ends of the legs 46 and 48 of the polepiece 18 and the ferromagnetic disc 22.

The voice coil 14 is cemented on the surface of the diaphragm 10 priorto assembly, and thereafter the diaphragm 10 is trimmed or cut to havean outer diameter approximately equal to the inner diameter of the ringportion 34 of the supporting ring 12 prior to assembly and coaxial withthe coil. Thereafter, it is only necessary to cement the flat peripheralportion 54 of the diaphragm on the surface of the ferromagnetic disc 22within the ring portion 34 of the supporting ring 12 to automaticallycenter the voice coil 14 in the gap 52.

In the construction of FIGURES l through 5, the pole piece 13 is in theform of `a U-shaped strip having legs 46 and 48 and a cross member 50.In the construction of FIGURE 6, the pole piece 18A is in the form of aferromagnetic -cup in which a cylindrical portion 46A replaces the legs46 and 48, and a flat bottom 50A replaces the cross member 50 of thefirst embodiment. Further, the flange 62 is provided with a pair ofoutwardly extending tabs 76 and 78 which carry a pair of lugs 80 whichreplace the lugs 20 of the prior construction. In other respects, theembodiment of the invention illustrated in FIGURE 6 is the same as thatillustrated in FIGURES l through 5.

It is of course intended that Ithe microphone structures here set forthshall be used in combination with a casing, designated 82 in FIGURE 3and a wind screen 84 to form a complete microphone. Depending upon thetype of microphone desired, the casing and wind screen may include otheracoustical or mechanical elements.

FIGURE 7 illustrates in electrical symbols the mechanical circuitemployed by the microphone structure. The acoustical force impressedupon the diaphragm 10 is indicated by the letter f. The inductance 86represents the mass of the moving system of the microphone, theresistance symbol 33 represents the mechanical resistance of the movingsystem, and Ithe capacitor symbol 9) represents the compliance of thediaphragm and suspension of the microphone structure. The capacitor 92represents the compliance of the chamber between the ferromagnetic disc22 and the diaphragm. The other three symbols in FIGURE 7 represent theeffect produced by the slots 44. More specifically, the inductancesymbol 94 represents the equivalent mass of the slots, the resistance 92represents the acoustical resistance of the slots 44, and the capacitor98 represents the compliance of the chamber on the side of thesupporting ring 12 opposite the diaphragm 10. By proper proportioning ofthese elements, the diaphragm may be damped to approach a resistancecontrolled microphone structure. Since the compliance of `the enclosedvolume of air within the pole piece 46A or within the casing 82 entersinto the damping of the diaphragm, these must be considered in designingthe microphone.

In one particular construction, the outer diameter of the diaphragm 10is approximately 1.06 inches and the diameter of the voice coil 14 isapproximately 0.50 inch.

The diameter of the opening 38 is approximately 0.58 inch and the innerdiameter of the recess 39 is approximately 0.535 rinch, the diameter ofthe arcuate surface of the slots 44 is approximately 0.528 inch, and thegap 52 has a width of approximately 0.0035 inch. The ferromagnetic disc22 has a thickness of approximately 0.07 inch and is constructed ofmagnet iron. The ring portion 34 of the supporting ring 12 has an outerdiameter of approximately 1.225 inches and an inner diameter ofapproximately 1.063 inches. The ring portion 34 extends from theferromagnetic disc 22 a distance of approximately 0.03 inch, and thedisc portion 32 has a thickness of approximately 0.108 inch. Thematerial of the nonmagnetic portion 22 of the supporting ring 12 ispolyethylene plastic. The diaphragm is also constructed of plastic andmay be Mylar.

FIGURE 8 fragmentarily illustrates a dynamic microphone which is amodification of the microphone illustrated in FIGURES l through 5.Identical reference numerals have been used in FIGURE 8 for elementswhich are identical to those in FIGURES 1 through 5. In this embodimentof the invention, the ferromagnetic disc 22A has no apertures, such asthe apertures 30 of the disc 22, for accommodating rod portions of thenonmagnetic part, but the disc 22A is in all other respects identicalwith the disc 22 `of the embodiment of FIGURES 1 through 5.

The nonmagnetic part 24A of FIGURE 8 has `a disc portion 32A with alarger diameter than the plate or disc 22A. A coaxial circular ange 90extends from the perimeter of the disc portion 32A about the perimeterof the disc 22A, and a flat ring portion 92 extends from the flange 90inwardly and in parallel abutment with the surface of the plate 22Aopposite the disc portion 32A of the nonmagnetic part 24A. In thismanner, the disc or plate 22A is clamped between the disc por-tion 32Aof the nonmagnetic portion 24A Iand the ring portion 92. The innerperiphery of the ring portion 92 is a cylindrical surface symmetricallydisposed about the gap 52, and the outer perimeter of the diaphragm 10abuts this surface and is -cemented on the surface of the disc 22A inthe manner of the embodiment of FIGURES l through 5.

It is to be noted that a pair of apertures 94 extend through the discportion 32A of the nonmagnetic part to accommodate the legs 46 of thepole piece in order to permit the legs to abut the disc of plate 22A andcomplete the magnetic circuit. Also, the nonmagnetic part may beplastic, as in the previous embodiments, and the nonmagnetic part may bemolded on the magnetic part, as previously described.

FIGURES 9, 10 and 11 illustrate an earphone constructed according to thepresent invention. As illustrated, the earphone is intended to bepositioned within a casing, but it is to be understood that theillustrated earphone is fully operative. As illustrated in the figures,a ring magnet 100 is disposed in abutment with a ferromagnetic disc 102which has a circular central aperture 104 therein. A cylindrical polepiece 106 of ferromagnetic material is disposed coaxially within thering magnet 100, and a generally circular ferromagnetic plate 108 ismounted adjacent to end of the pole piece 106 opposite the disc 102. Theplate 108 has a plurality of apertures l112 therethrough disposed alonga circular path coaxially disposed Iabout lthe opening 110 in the plate108.

The entire assembly of magnet 100, disc 102, pole piece 106, and plate108 are held together in a unitary assembly by a casing 114 of plasticmaterial, such as nylon. The casing 114 has a flat bottom 116 disposedin abutment with the disc 102, a cylindrical wall 118 extending from theflat bottom in abutment with the exterior surface of the ring magnet100, and a cylindrical sleeve 120 disposed between the ring magnet 100and the pole piece 106 abutting these elements to form a spacer membertherebetween. In addition, the casing has a ring portion 122 disposed inabutment with the surface of the plate 108 opposite the ring magnet andcoaxial with the ring magnet 100. The ring portion 122 is connected tothe other portions of the casing by posts 124 to form an integralplastic body. The plate 108 has a recess at the periphery thereof, andthis recess contains a protrusion 126 of the casing, and two terminallugs 128 are embedded within the protrusion of the casing 114.

A damping plug 130 is mounted within the inner cylindrical surface ofthe pole piece 106 and has a hollow sleeve 132 which abuts the polepiece 106 and may be cemented thereto. The plug 130 also has a cap 134with an opening 136, and the opening 136 is covered by a layer of cloth138.

A diaphragm 140 with a circular perimeter of approximately the samediameter as the inner diameter of the ring portion 122 of the casing 114is mounted within the ring portion 122 on the plate 108. The diaphragmis similar in construction to the diaphragm 10 previously described, andhas a peripheral portion 54, a flange 57, and a dome 58, but the flutedportion 56A between the dome 58 and the flange 57 has flutes disposedalong radii of the diaphragm 140. In the embodiment of FIGURES 1 through5, the flutes are disposed on axes tangential to the voice coil 14. Inthe construction of FIGURES 1 through 5 the flutes provide additionalcompliance to the diaphragm 10, which is desirable for a microphoneapplication. The flutes of the diaphragm 140 form stiifening ribs andreduce the compliance of this diaphragm, which is desirable in anearphone construction.

It is to be noted that the voice coil 14 is also mounted on thediaphragm 140 in an identical manner to that illustrated in FIGURES lthrough 5 and disposed in a magnetic gap formed between the pole piece106 and the plate 108. Also, the sleeve 120 of the casing 114 has acircular recess 142 extending from the plate 108 to provide adequateroom for translation of the voice coil. The voice coil leads are broughtout from the coil 14 between the diaphragm 140 and the plate 108 to thelugs 128 mounted in the protrusion 126.

FIGURE 12 illustrates a cavity mold for injection molding the casing 114and simultaneously mounting the disc 102, magnet 100, pole piece 106 andplate 108. The mold has an upper part and a lower part 152 whichtogether form a cavity 154 with the contour of the exterior surface ofthe casing 114. In addition, there is a passage 156 extending throughthe upper portion of the mold for injection of plastic into the cavity154. The mold also has a cylindrical sleeve 158 extending upwardly fromthe lower portion of the mold centrally within the cavity, and acylindrical boss 160 extending from the lower portion of the moldcoaxially within the sleeve 158.

To utilize the mold, the plate 108 is placed against the lower portionof the mold with the opening 110 thereof disposed about the sleeve 158.Also, the pole piece 106 is mounted coaxially between the sleeve 158 andthe boss 160 of the lower portion of the mold. Next, the ring magnet 100and the disc 102 are positioned on the plate 108, and thereafter theupper portion of the mold is closed over the lower portion of the mold.

The upper portion of the mold is provided with four pins 162 which matewith four openings 164 in the disc 102. The pins are positioned to alsoabut the interior surface of the ring magnet 100. As a result, the pins162 hold the ring magnet 100 and the disc 102 in xed position within themold. Plastic, in liquid phase, may then be injected into the cavityunder pressure through the passage 156 to form the casing 114 whensolidified, and simultaneously assemble the magnet 100, disc 102, polepiece 106, and plate 108 in a single operation.

After removing the assembly from the mold, the plug 130 is cemented intothe pole piece 106. Thereafter, the assembled diaphragm and voice coilare cemented into place as indicated in connection with the embodimentof FIGURES 1 through 5. The leads from the voice coil are also connectedto the terminal plugs 128 before the diaphragm 140 is cemented inposition.

It is well known that materials and dimensions other than thoseillustrated may also be employed. Further, those skilled in the art willreadily devise many modifications of the present invention andapplications of the present invention beyond those here set forth. It istherefore intended that the scope of the present invention be notlimited by the foregoing disclosure, but rather only by the appendedclaims.

The invention claimed is:

1. An electroacoustical device comprising a support member including aferromagnetic plate having a cylindrical opening and a nonmagneticmember mounted on the plate having a first portion on one side of theplate, la second portion on the opposite side of the plate and aninterconnecting portion extending from the irst portion to the secondportion, .the first portion having an orice with a central axis on theaxis of the opening in the plate, and the second portion forming a ribextending about the opening in the plate iand having a wall confrontingthe opening, a magnetic circuit structure including the plate and aferromagnetic member having a central axis disposed on the axis of theopening in the plate, said member abutting -the orifice in the firstportion of the nonmagnetic member -and having a cylindrical portion witha smaller diameter than the opening in the plate disposed coaxiallywithin the opening, a diaphragm having a perimeter with the contour ofthe wall of the rib of the second portion of the nonmagnetic member,said diaphragm being disposed within the rib and mounted on the supportmember, and a cylindrical voice coil mounted on the diaphragm anddisposed within the opening between the plate and the cylindricalportion of the ferromagnetic member.

2. An electroacoustical device comprising the elements of claim 1 incombination with an air impermeable casing acoustically sealed on thesupport member and surrounding the magnetic circuit structure.

3. An electroacoustical device comprising the elements of claim 1wherein the magnetic circuit structure includes a ferromagnetic yokehaving a first portion abutting the end of the rod opposite the plateand a pair of leg por- 'tions extending from opposite sides of the rstportion and abutting the plate at opposite sides of the nonmagneticmember.

4. An electroacoustical device compris-ing the elements of claim 1wherein the magnetic circuit structure includes a ferromagnetic cuphaving an opening at one end disposed about the rst portion of thenonmagnetic member in abutment with the plate, said cup having a closedend in abutment with the end of the rod opposite the plate.

5. An electroacoustical device comprising the elements of cl-aim 1wherein the magnetic structure includes a ring magnet disposed coaxiallyabout the member and having one pole thereof in abutment with theferromagnetic plate, a second ferromagnetic plate abutting the otherpole of the ring magnet and a portion of the member remote from the rstplate.

6. An electroacoustical device comprising the elements of claim 5wherein .the member is provided with a channel extending therethrough,and a plug is disposed within the channel having a porous member toprovide acoustical damping.

'7. An electroacoustical dev-ice comprising the elements of claim 5 incombination with a casing secured to the rst plate and extending aboutthe ring magnet and member on the side of the plate remote from thediaphragm, said casing being of plastic material and integral with thenonmagnetic member.

8. An electroacoustical dev-ice comprising a magnetic circuit includinga source of magnet-omotive force, a ferromagnetic plate having a flatsurface and a cylindrical opening extending through the plate normal tosaid surface, a ferromagnetic rod having a cylindrical portion ofsmaller diameter than the opening disposed coaxially within the opening,a diaphragm having a at portion extending outwardly from a circularcentral portion, said flat portion being disposed with one surfacethereof mounted on said surface of the plate and disposed coaxiallywiththe opening of the central portion, said diaphragm having agenerally truncated conical flange portion extending from the inner sideof the at portion coaxially with the opening at an angle to the atcircular portion and away from the plate, the central portion of saiddiaphragm having an inner portion extend-ing inwardly from the flangeportion and spaced from the plate, and a cylindrical voice coil mountedon the central por- -ti-on of the diaphragm coaxial with the opening andtranslatably disposed in the opening between the plate and thecylindrical portion of the rod.

9. An electroacoustical device comprising the combination of claim 8wherein the flat surface of the ferromagnetic plate is provided with anoutwardly extending rim and the outer edge of the flat portion of thediaphragm abuts the rim.

1t). An electroacoustical device comprising the combination of claim 8wherein the flat portion of ythe diaphragm has a circular outer edgecoaxially disposed about the voice coil, and the at surface of theferromagnetic plate is provided with a circular rim extending coaxiallyabout the opening of the plate, said rim having an inner surfacedisposed in abutment with the outer edge of the fiat portion of thediaphragm.

11. An electroacoustical device compris-ing the elements of claim Swherein lthe inner portion of the diaphragm comprises a circular domecentered on the axis of the opening in the plate, said dome having adiameter approximately equal to the diameter of the opening in the plateand extending away from the plate.

12. An electroacoustical device comprising the elements of claim 11wherein Ithe inner portion of the diaphragm includes a second truncatedconical portion between the dome and the ange portion, said secondtruncated conical portion sloping toward the surface of the plate.

13. An electroacoustical device comprising the elements of claim 8 incombination with a pair of lugs mounted on the plate and electricallyinsulated from each other, the diaphragm having an aperture thereinadjacent to the voice coil and the voice coil having a pair of wireleads extend-ing through the aperture and electrically connected to thelugs.

14. An electroacoustical device comprising the elements of claim 12wherein the voice coil is mounted on the surface of the dome adjacent tothe outer edge of the dome.

15. An electroacoustical device comprising the elements of claim 12wherein the second truncated conical portion of the inner portion of thediaphragm is provided with a plurality of futes which extend about thedome.

16. An electroacoustical device comprising the elements of claim 15wherein each of the flutes has a straight ridge disposed tangentially ofa circle centering on the center of the inner portion.

17. An electroacoustical device comprising the elements of claim 15wherein each of the flutes has a straight ridge disposed on one of theradii of the flange portion.

18. A dynamic microphone comprising a support member including aferromagnetic plate hav-ing a cylindrical opening and a nonmagneticmember constructed of plastic mounted on one side of the plate andhaving a cylindrical orifice coaxial with the opening, said nonmagneticplastic member having a plurality of spaced ribs disposed parallel tothe axis of the opening and extending inwardly from the orifice, saidribs terminating in surfaces disposed at a common distance from the axisof the orifice, a magnetic circuit structure including the plate and acylindrical rod disposed within the orifice of the nonmagnetic member,said rod having an outer radius slightly greater than the distancebetween the surfaces of the ribs and the axis of the orifice and beingwedged between the ribs of the nonmagnetic member and supported by saidribs, said rod having a portion disposed within the opening of the plateto form a circular gap with the plate, a ferromagnetic yoke extendingfrom a portion of the rod remote from the plate and abutting the plate,a diaphragm mounted on the side of the plate opposite the nonmagneticmember, and a cylindrical voice coil disposed within the gap between therod and the plate and mounted on the diaphragm.

19. A dynamic microphone comprising, in combination: a support ringincluding a circular ferromagnetic disc having a central circularopening and a plurality of apertures extending therethrough centered ona circle disposed coaxially about the opening and spaced from each otherby equal distances, and a unitary nonmagnetic part mounted on theferromagnetic disc having a ring portion disposed on one side of thedisc confronting the coaxial axis of 4the apertures and having acylindrical inner surface coaxial with the opening, a disc portionabutting the side of the disc opposite the ring portion with an outersurface disposed on the side of the apertures remote from the opening,said disc portion having a cylindrical orifice coaxial with the open-ingin the disc and of the same diameter as the diameter of the opening inthe disc and a plurality of ribs extending inwardly from the cylindricalsurface of the orifice parallel to the axis thereof and terminating insurfaces disposed at a common distance from the axis of the opening, andpost portions extending through each of the apertures from the ringportion to the disc portion; a cylindrical magnet having a radius equalto 4the common distance between the surfaces of the ribs and the axis ofthe opening disposed within the opening in the disc and orifice in thedisc portion, the ends of said magnet being on fiat planes normal to thecentral axis of the magnet and one end of said magnet being on the planeof the surface of the disc adjacent to the ring portion, thereby forminga gap between the magnet and d-isc; a ferromagnetic yoke having a flatportion abutting the flat end of the magnet opposite the disc and aportion extending from the flat portion of the yoke to abut theferromagnetic disc; a diaphragm constructed of thin compliant materialhaving a circular periphery with the diameter of the inner surface ofthe ring port-ion of the support ring, said diaphragm having a flatperipheral portion disposed within the ring portion of the support ringand abutting the surface of the ferromagnetic disc, said diaphragm alsohaving a circular band of utes disposed within the flat portion andextending away from the disc and a circular dome extending away from thedisc coaxially with the opening therein; and a cylindrical voice coilmounted on the diaphragm coaxial with the periphery of the diaphragm,

said voice coil having a diameter between the diameter of the opening inthe disc and the diameter of the magnetic and being translatablydisposed in the gap between the disc and the magnet.

20. A subassembly for an electroacoustical device compris-ing a ringmagnet, a cylindrical ferromagnetic pole piece disposed within the ringmagnet, a first ferromagnetic plate having a circular opening coaxiallydisposed about the pole piece forming a magnetic gap and extending toabut one pole of the ring magnet, a second plate abutting the other poleof 4the ring magnet and the pole piece remote from the gap, and anonmagnetic casing having a sleeve disposed between the ring magnet andthe pole piece, said casing being secured to the first plate andextending about the ring magnet and pole piece to secure the pole piece,ring magnet, first and second plates into an assembly.

21. A subassembly for an electroacoutical device cornprising theelements of claim 20 wherein the Pole p-iece has a channel extendingtherethrough, a plug disposed within the channel having a porous memberto provide acoustical damping, and a diaphragm sealed on the first plateabout the plug.

22. An electroacoustical device comprising the combination of claim 20-in combination with a diaphragm having a flat portion with an innercircular region mounted on the first plate remote from the second plate,the perimeter of the circular region being coaxially disposed about thecylindrical pole piece, said diaphragm having a flange portion withinthe flat portion extending at an angle from the first plate, and saiddiaphragm having a central portion extending from the ange portion, anda cylindrical coil mounted on the central portion coaxial with thecylindrical pole piece and translatably disposed in the magnetic gap.l

References Cited by the Examiner UNITED STATES PATENTS 1,766,473 v6/1930 Wente 179-115.5 2,442,791 6/ 1948 Wente 181-31 2,490,227 12/ 1949Murkham 179-115.5 2,773,130 12/1956 Olson et al. 179-115.5 2,773,93312/1956 Hawley 179-1 2,775,653 12/1956 Wurdel 179-115.5 2,862,06911/1958 Marchard et al. 179-115.5 2,950,359 8/1960 Perry 179-115.53,033,945 5/1962 Villchur 179-115.5 3,061,690 10/ 1962 Swinehart 179-1ROBERT H. ROSE, Primary Examiner. H, W. GARNI-ER, F. N, CARTEN,Assistant Examiners.

UNITED STATES PATENT OFFICE CERTIFICATE 0F CORRECTION Patent No.3,258,543 June Z8, 1966 Harold P. Moser, Jr.

It is certified that error appears in the above identified patent andthat said Letters Patent are hereby corrected as shown below:

Column 8, line 6, "opening of the central portion," should read opening,the central portion of line S6, "futes" should read flutes Signed andsealed this 16th day of December 1969.

(SEAL) Attest:

WILLIAM E. SCHUYLER, JR.

Edward M. Fletcher, Jr.

Commissioner of Patents Attesting Officer

1. AN ELECTROACOUSTICAL DEVICE COMPRISING A SUPPORT MEMBER INCLUDING AFERROMAGNETIC PLATE HAVING A CYLINDRICAL OPENING AND A NONMAGNETICMEMBER MOUNTED ON THE PLATE HAVING A FIRST PORTION ON ONE SIDE OF THEPLATE, A SECOND PORTION ON THE OPPOSITE SIDE OF THE PLATE AND ANINTERCONNECTING PORTION EXTENDING FROM THE FIRST PORTION TO THE SECONDPORTION, THE FIRST PORTION HAVING AN ORIFICE WITH A CENTRAL AXIS ON THEAXIS OF THE OPENING IN PLATE, AND THE SECOND PORTION FORMING A RIBEXTENDING ABOUT THE OPENING IN THE PLATE AND HAVING A WALL CONFRONT INGTHE OPENING, A MAGNETIC CIRCUIT STRUCTURE INCLUDING THE PLATE AND AFERROMAGNETIC MEMBER HAVING A CENTRAL AXIS DISPOSED ON THE AXIS OF THEOPENING IN THE PLATE, SAID MEMBER ABUTTING THE ORIFICE IN THE FIRSTPORTION OF THE NONMAGNETIC MEMBER AND HAVING A CYLINDRICAL PORTION WITHA SMALLER DIAMETER THAN THE OPENING, A DIAPHRAGM HAVING A COAXIALLYWITHIN THE OPENING, A DIAPHRAGM HAVING A PERIMETER WITH THE CONTOUR OFTHE WALL OF THE RIB OF THE SECOND PORTION OF THE NONMAGNETIC MEMBER,SAID DIAPHRAGM BEING DISPOSED WITHIN THE RIB AND MOUNTED ON THE SUPPORTMEMBER, AND A CYLINDRICAL VOICE COIL MOUNTED ON THE DIAPHRAGM ANDDISPOSED WITHIN THE OPENING BETWEEN THE PLATE AND THE CYLINDRICALPORTION OF THE FERROMAGNETIC MEMBER.